C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  precedence question          C++Talk.NET Forum Index -> C++ language (comp.lang.c++) View previous topic :: View next topic   Author Message gouqizi.lvcha@gmail.com Guest Posted: Fri Jul 29, 2005 7:59 pm    Post subject: precedence question I have a statement as follows, a = b++; why b=b+1 after a=b. I check the C language precedence (K&R Page 52) , ++ should has higher precedence than = . Rick Back to top Tobias Blomkvist Guest Posted: Fri Jul 29, 2005 8:11 pm    Post subject: Re: precedence question [email]gouqizi.lvcha (AT) gmail (DOT) com[/email] sade: Quote: I have a statement as follows, a = b++; why b=b+1 after a=b. I check the C language precedence (K&R Page 52) , ++ should has higher precedence than = . Rick Perhaps you should check the *C++ Standard* instead. Tobias -- IMPORTANT: The contents of this email and attachments are confidential and may be subject to legal privilege and/or protected by copyright. Copying or communicating any part of it to others is prohibited and may be unlawful. Back to top Tobias Blomkvist Guest Posted: Fri Jul 29, 2005 8:23 pm    Post subject: Re: precedence question Pete Becker sade: Quote: Doesn't matter: they both require that behavior. I wasn't implying anything else. Tobias -- IMPORTANT: The contents of this email and attachments are confidential and may be subject to legal privilege and/or protected by copyright. Copying or communicating any part of it to others is prohibited and may be unlawful. Back to top msh Guest Posted: Fri Jul 29, 2005 10:05 pm    Post subject: Re: precedence question use ++b Back to top msh Guest Posted: Fri Jul 29, 2005 10:06 pm    Post subject: Re: precedence question use ++b Back to top Alan Johnson Guest Posted: Sat Jul 30, 2005 1:32 am    Post subject: Re: precedence question [email]gouqizi.lvcha (AT) gmail (DOT) com[/email] wrote: Quote: I have a statement as follows, a = b++; why b=b+1 after a=b. I check the C language precedence (K&R Page 52) , ++ should has higher precedence than = . Rick Let's define a new operator, @. @ has the following semantics. The expression @x evaluates to the value x+1. The expression x@ evaluates to the value of x. In neither case does the value of x change. Now, let's let x = 3. What are the values of x, y and z after the following assignments? y = @x ; z = x@ ; Obviously, x = 3, y = 4 and z = 3. The only difference between the @ operator I made up, and the ++ operator is that ++ has a side effect of incrementing the variable to which it is applied. It still holds that ++x evalutes to the value x+1, and x++ evalutes to the value of x. Now, to answer your question, "a=b" doesn't happen anywhere, never, not at all, not before "b++", and not after "b++". What happens is that, first, the expression "b++" is evaluated. This expression evaluates to whatever the value of b is (before incrementing). It makes no difference that as a side effect b then gets incremented. So then, the result of of the expression "b++" (which is b's old value) gets assigned to a. Back to top Ron Natalie Guest Posted: Sun Jul 31, 2005 12:26 pm    Post subject: Re: precedence question [email]gouqizi.lvcha (AT) gmail (DOT) com[/email] wrote: Quote: I have a statement as follows, a = b++; why b=b+1 after a=b. I check the C language precedence (K&R Page 52) , ++ should has higher precedence than = . Precedence is a different concept from order of evaluation. Further, order of evaluation is a different concept than the rules for updating variables in an expression. Precedence is the rules for interpreting a statement. That ++ has a higher precedence than = just means that the interpretation of the above is: a = (b++) and not (a = b) ++ C++ doesn't mandate any particular order of evaluation in most cases. The compiler is free to reorder the processing of subexpressions to make things optimal. Further until you hit a sequence point (which above is at the end of the full expression), there's no guarantee of when the variables will be changed. Anyhow, in your case the expression b++ is defined to be the value of b BEFORE the increment. The value of ++b is the value before the increment PLUS 1. There's no order of evaluation involved here, just the meaning of the operators. Back to top Pete Becker Guest Posted: Mon Aug 29, 2005 8:13 pm    Post subject: Re: precedence question Tobias Blomkvist wrote: Quote: gouqizi.lvcha (AT) gmail (DOT) com sade: I have a statement as follows, a = b++; why b=b+1 after a=b. I check the C language precedence (K&R Page 52) , ++ should has higher precedence than = . Rick Perhaps you should check the *C++ Standard* instead. Doesn't matter: they both require that behavior. -- Pete Becker Dinkumware, Ltd. (http://www.dinkumware.com) Back to top Pete Becker Guest Posted: Mon Aug 29, 2005 8:14 pm    Post subject: Re: precedence question [email]gouqizi.lvcha (AT) gmail (DOT) com[/email] wrote: Quote: I have a statement as follows, a = b++; why b=b+1 after a=b. I check the C language precedence (K&R Page 52) , ++ should has higher precedence than = . Read the description of what postfix ++ does. -- Pete Becker Dinkumware, Ltd. 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